The envelope glycoprotein of HTLV-III/LAV is likely to possess important biological properties, including the ability to interact with receptors on susceptible cells and to induce cell fusion. It is also probably the major target for induction of protective immunity against virus infection. This proposal is designed to employ molecular genetic and immunological approaches to provide a better understanding of the structure and biosynthesis of this glycoprotein, as well as its antigenic properties. Recombinant vaccinia viruses will be employed as expression vectors for investigation of the biosynthesis and processing of the glycoprotein, as well as its orientation in cellular membranes. The gene encoding the envelope glycoprotein will be modified in an effort to produce a secreted form of the glycoprotein, which will be expressed in eukaryotic cells with the use of recombinant vaccinia virus or bovine pappiloma virus-derived vectors. The secreted form of the viral glycoprotein will be purified by standard techniques for protein fractionation, and used to prepare a panel of monoclonal antibodies to the HTLV-III/LAV glycoprotein. These antibodies will be classified according to their patterns of reactivity in competition binding experiments, as well as their ability to interact with fragments of the glycoprotein obtained by controlled proteolysis. The effects of monoclonal antibodies on viral biological activities, including infectivity and cell fusion activity, will also be determined. Based on recent findings of extensive variation in the envelope gene of HTLV-III/LAV, and the fact that antigenic variation is known to occur in related viruses such as visna and equine infectious anemia virus, we will utilize the monoclonal antibodies described above to undertake a study of antigenic variation in the viral glycoprotein. The results of these investigations will be correlated with studies being carried out at UAB by Drs. George Shaw and Beatrice Hahn concerning the nucleotide sequence variation in the envelope gene of HTLV-III/LAV.